JP-A-2002-10685 (patent document 1) discloses a related art time compensation control for a motor driven by an inverter. The motor control device of this publication provides a compensation amount for a dead time occurring due to PWM control to an PWM signal for driving an inverter. According to this motor control device, a dead time compensation gain is calculated in accordance with a torque amount calculated on the basis of a torque current instruction value, and the compensation gain thus calculated is multiplied by the compensation amount described above to correct the PWM signal. Accordingly, it is possible to smoothen the PWM signal to be transmitted to the inverter, so that it is possible to suppress the current variation of the motor and suppress the torque variation of the motor.
In addition to the above patent document 1, JP-A-2000-85610 (patent document 2) discloses another related art in which a steering device is equipped with a transmission ratio varying mechanism for varying the transmission ratio between a steering angle of a steering wheel and a turning angle of a steered wheel, and the transmission ratio varying mechanism is applied to a motor.
If some dispersion exists in electrical characteristic values of switching elements provided to the inverter, even when a dead time compensation gain is settled and the compensation amount of the dead time is multiplied by the gain to correct the PWM signal, the wave height and waveform of a sine wave of each phase generated by the inverter are not coincident with those of a reference sine wave, surge occurs in torque current and torque variation occurs.
Furthermore, when the motor disclosed in the patent document 2 is driven by using the above motor controller, torque variation occurs in the motor for the above reason, and this torque variation is transmitted to the steering wheel, so that a driver may have an uncomfortable feeling. The same symptom may occur in an electrical power steering in which an auxiliary mechanism for assisting the steering force of a steering wheel is applied to a motor.